On Settlements in Urban Areas with Soft Clay - As a result of leakage to deep excavations via an underlying permeable layer

Abstract

The need of infrastructure, such as tunnels, increases as cities grow and densify. When tunnels are constructed in soil, open cut excavations are often used. During the time when the excavation is open, there will be a hydraulic head difference between the inside and outside of the excavation. Water can then flow into the excavation which in turn results in reduced pore pressures and ground settlements on the outside. The excavation analysed in this project is a part of the West link, a train tunnel that is being built to shorten travel times throughout the Gothenburg region. The soil in the area around the excavation consists of a thick layer of low permeable clay, underlain by a few metres of friction material followed by bedrock. Most of the inflow is expected to occur via the friction layer. The pore pressure reduction will thus initially take place in the friction material and the clay is over time affected by consolidation. The area which is affected by an eventual pore pressure reduction is potentially very large and it is out of the scope of this study to determine the size of the area. Additionally, it is basically impossible to create a completely trustworthy groundwater model. Instead, the relation between pore pressure reduction, time, and settlement is investigated to give an indication of when problems start to occur. By continuously measuring the pore pressures in the friction layer in the area during the construction time, it is possible to detect any deviations. Hopefully then can prevention measures be taken before significant settlements occur. In this thesis, a model representative for the site was created in the numerical software Plaxis 2D to study the relation between pore pressure reduction, time, and settlement. The soil characteristics was evaluated from various soil tests which previously had been performed on behalf of Trafikverket. Two different constitutive models were used in Plaxis: Soft Soil and Soft Soil Creep. The latter was proven to represent the expected behaviour of the clay during pore pressure reductions much better than the Soft Soil model. The most significant results in this project is from the Soft Soil Creep model and it shows that the settlements increase exponentially with increasing pore pressure reductions. It takes for example 21 days until the settlements have reached 50 mm after a pore pressure of 60 kPa. The corresponding number for a pore pressure reduction of 10 kPa is 1100 days. There are several uncertainties regarding the results. The results should therefore not be interpreted as facts, but rather give a hint of when settlement issues may arise. The damage to buildings due to settlements can become large. It is therefore important to seal the excavation as good as possible and to prepare solution measures if leakage after all occurs.